Turbines are often erected in groups, or wind turbine parks, and operated synchronized to mimic the behaviour of a power plant. In wind turbine parks comprising multiple wind turbines, the individual wind turbines operate in a complex aerodynamic environment due to the unsteady, turbulent nature of the wind field. Additionally, as the turbines extract power from the wind, they produce a wake behind them of generally lower wind speed and of a higher turbulence rating. Due to space considerations the distance between the individual turbines is often such that turbines are in the wind-shadow or wake of other turbines for specific wind directions. All turbines operating in a wake will encounter lower wind speeds than the ambient wind speed outside the plant, and more turbulent wind. This results in a lower performance, and a higher fatigue loading. This may be seen as a reduced total power production of the wind park compared to the total power to be expected from the same number of individual turbines placed separately. The primary “wake loss”, designated as the difference between the power curve of an individual turbine and the mean or median park power curves, occurs in the partial power region of the operational regime (usually at wind speed between 3-12 m/s).
Additionally, the wake loss is seen to depend on the state of the atmospheric boundary layer. Depending upon the ambient temperature distribution in the atmosphere and time of day, the atmosphere can either be classified as neutral, stably stratified, or convectively unstable. The evolvement of the wake downstream depends on the state of the atmosphere, and therefore the power distribution along rows of turbines also is very sensitive to the state of the atmosphere. It has be seen that there may be approximately a 20% difference in downstream performance with different atmospheric conditions.
While the concept of a “row” of wind turbines is straightforward, often the wind sector forces turbines to operate in an unstructured mode, where rows of well aligned turbines cannot be defined. In such wind directions, situations where one turbine is partially waking a downstream turbine will occur.
For larger wind turbine parks, wake losses are inevitable but the siting process most often aims at minimizing the wake losses in the wind directions offering the most wind energy by the positioning of the turbines. However, as turbines cannot be moved, wind shadowing will eventually occur in some wind directions, causing wake losses. Even though wake losses cannot be avoided for larger farms with turbines erected relative close to one another, the magnitude of the losses can be affected by choice of the operating strategy for the individual turbines.
Today, the operation strategy for the turbines is often focused on optimizing the individual turbines according to the local experienced wind speed and wind direction. Also, much effort over the years have been focused on optimizing the individual turbines to produce as much power as possible taking into account the variation of the local wind conditions and the internal conditions in the turbine (such as oil pressure and converter temperatures). However, as the possible productions of the individual turbines in a park are in some situations dependent on one another, it is not given that the overall production of the park is the highest when each turbine produces as much power as possible under the given wind conditions. Rather, it has been proven that in some cases restricting the power production on some turbines leads to a higher power production of the entire farm.
Further, in some wind turbine parks, the wind turbines are controlled and regulated by inter-turbine coordination through communication and from one central processing unit. Known control methods for wind turbine parks include collecting data row-wise from each wind turbine of the park at a central processing or control unit which from these data determines potential wake conditions for some of the turbines. For each row some optimization calculations may then be performed to determine the optimal power production for the row, and adjusted control signals are then transmit to each turbine in each row to reduce the power output of selected upstream turbines. The controlling is thereby dependent on the central control unit and on the communication between the turbines which makes the system more vulnerably, complex, and involves a certain non-negligible response time or delay in the controlling.